1. Field of the Invention
The present invention relates to an echo canceling system in a full-duplex communication system.
2. Description of Related Art
Accompanying a widespread use of the internet, various types of services utilizing the internet have started. For example, what is called a computer telephony system using a technology such as VoIP via the internet has become common. The following is a description of a conventional full-duplex communication system using the VoIP application via the internet.
FIG. 13 is a diagram schematically showing the full-duplex communication system. In FIG. 13, numerals 10 and 20 both denote speakers. Hereinafter, “speaker” means a person who holds a conversation by using the computer telephony, and “loudspeaker” means a device of the computer telephony outputting the transmitted voice. For convenience, the following description is directed to an example of the case where the speaker 10 speaks and the voice of this person is transmitted to the speaker 20 side. Numerals 11 and 21 denote microphones, numerals 12 and 22 denote loudspeakers, numerals 13 and 23 denote VoIP applications, numerals 14 and 24 denote terminals, and numeral 30 denotes the internet. A communication interface and other devices are omitted in this figure for convenience.
When the speaker 10 inputs voice to the microphone 11, the VoIP application 13 receives a voice signal corresponding to this voice and performs necessary processings such as a sampling, so as to be transmitted from the terminal 14 to the internet 30 as packet data. All packet data follow respective routes on the internet and reach the terminal 24. These packet data are assembled sequentially and subjected to necessary processings by the VoIP application 23, then outputted from the loudspeaker 22 as voice.
During such processings, it has been known conventionally that a phenomenon called an echo occurs. The voice outputted from the loudspeaker 22 not only reaches the conversation partner 20, but sometimes is re-inputted undesirably to the microphone 21 as voice. In this case, the voice re-inputted from the microphone 21 is transmitted via the VoIP application 23 in the terminal 24, the internet 30 and the VoIP application 13 in the terminal 14, thus being outputted from the loudspeaker 12 as voice. This forms a kind of loop. In a path that forms this loop generating the echo (hereinafter, referred to as “an echo path”), transmission delay is generated. In other words, the speaker 10 hears the voice that he/she inputted to the microphone 11 from the loudspeaker 12 a little later. When this echo occurs, it is known that the speaker finds it very difficult to talk and listen to the voice of the partner. Also, when the echo level is very high so that the echo diverges without fading, a phenomenon called howling occurs. In this case, it is very difficult to have a conversation.
In response to this, an echo canceller has been used conventionally. FIG. 14 is a diagram schematically showing an echo canceling system using the conventional echo canceller. In FIG. 14, a terminal 24 on a conversation partner 20 side has an echo canceller 25. The echo canceller 25 receives a signal to be outputted via a loudspeaker 22 as an input and subtracts the received signal from a signal captured by a microphone 21, thereby canceling a voice signal that has been undesirably headed and re-inputted in the microphone 21.
As illustrated in FIG. 14, the conventional echo canceller has had a configuration that the voice signal undesirably head from the loudspeaker toward the adjacent microphone is canceled on the spot, thus preventing this undesirably received voice signal from returning to a system on the speaker side.
However, the echo canceling system using the conventional echo canceller has the following problems.
First, an echo canceller is not always installed and operated in a system on the conversation partner side. In the conventional echo canceling, it is necessary to cancel out the undesirable path of the voice between the loudspeaker and the microphone in the system on the partner side (hereinafter, referred to as “a far-end undesirable path”). For this purpose, the echo canceller has to be installed and operated in the system on the partner side. However, this is not always expected. If it is not installed and operated, the speaker has to ask the partner to install and operate the echo canceller.
Second, when performing an echo canceling processing, it is necessary to consider not only canceling out the echo caused by the far-end undesirable path but also canceling out an echo caused by the undesirable path of the voice between the loudspeaker and the microphone in the system on the speaker side (hereinafter, referred to as “a near-end undesirable path”). The voice of the conversation partner and the voice in the case where the far-end undesirable path is not completely canceled out are outputted from the loudspeaker on the speaker side. The near-end undesirable path caused by the input of those voices from the loudspeaker to the microphone also has to be prevented.
Third, the speaker, who will actually hear the echo, cannot make fine adjustments to the echo canceling processing for him/herself. In a conversation, the echo causes a problem that the speaker finds it difficult to talk and listen because of his/her own echo voice outputted from the loudspeaker. Accordingly, how the speaker feels the echo is subjective. Therefore, it is preferable that, in the echo canceling processing, the speaker can adjust a hearing condition of the echo for him/herself. However, in the conventional echo canceling system, since the echo canceller is located in the system on the partner side, the speaker cannot adjust the echo canceller according to the hearing condition of the echo. Although it is possible technologically to operate parameters of the echo canceller in the system on the partner side by remote control, the partner side cannot accept such environment from the viewpoint of security.
Such a problem also means that a user who has introduced the echo canceller in his/her own system cannot perceive the effect of it directly. In other words, the user who has introduced the echo canceller can neither feel benefit of an echo canceling effect by the introduction, nor adjust the echo canceller in order to achieve better echo canceling effect. The echo canceller is introduced not for him/herself but for partner side. Thus, even when a user introduces it, its cost and the benefit for the user are not balanced. This is one of the obstacles of the diffusion of the echo canceller.
Fourth, when voice signals are exchanged by packet telecommunication via the internet, the condition of the echo that is heard actually by the speaker varies easily. The echo is influenced by a transmission condition of a network. In a transmission of the voice signals by packet telecommunication via the internet, the delay amount of a network transmission is large. Also, the condition of the network transmission varies easily because of the routing and the variation of traffic amount, leading to a large characteristic fluctuation when transmitting the voice signals. Therefore, the characteristic amount cannot be determined simply from the distance between the speakers and will fluctuate even if it is calculated once. Consequently, the mere installation of the conventional echo canceller on the speaker side does not make it possible to obtain an appropriate signal for canceling the echo in a reliable manner.